Type 1 diabetes mellitus (T1DM) is a chronic autoimmune disorder that presents a significant set of challenges to the child, their family and the interdisciplinary team of health professionals. Medical nutrition therapy is an essential component of education for children with T1DM. However, there are gaps in the evidence regarding the optimal approach to dietary management of children and adolescents using intensive insulin therapy, including the precision required in carbohydrate counting to maintain glycaemic control; the ability of children and their families to accurately count carbohydrate; and the impact of errors in carbohydrate quantification on postprandial glycaemia. The primary purpose of this thesis is to investigate the effect of variations in carbohydrate quantity on postprandial glycaemia, and the ability of children and their families to estimate carbohydrate using different quantification methods. The results of the national survey on the dietary management of children and adolescents on insulin pump therapy highlighted diversity in clinical dietetic practice. Overall, a lack of evidence and consensus was identified with regard to the degree of precision required in carbohydrate counting estimations. Furthermore, limitations exist in the accuracy of the nutrition information panel on a food label, which has direct implications for clinical practice. The optimal method of quantifying carbohydrate (one gram increments, 10 gram portions or 15 gram exchanges) remains a controversial issue. A questionnaire conducted in clinics in Australia and the UK that examined the ability of children and their parents to count carbohydrate, demonstrated that 73 percent of all estimates (n=2530) were within a 10-15gram error margin, no matter which method of estimation was used. This study showed that children and their parents can quantify carbohydrate in meals with reasonable accuracy, provided education is given by experienced health professionals. The carbohydrate variation studies were undertaken to assess the impact of 10 gram and 20 gram variations in carbohydrate amount of a standardised meal for a set insulin dose. The studies demonstrated that insulin covers a range in carbohydrate amounts, and that a 10 gram variation in carbohydrate estimations for a meal containing 60 grams of carbohydrate does not make a difference to postprandial glucose levels, but that a 20 gram variation results in significant postprandial hypoglycaemia and hyperglycaemia. Overall, this sequence of studies seeks to improve the effectiveness of medical nutrition therapy related to premeal insulin adjustment for carbohydrate amount. The clinical implications of the findings presented in this thesis are discussed and specific recommendations offered for practice and research in order to facilitate improved outcomes for children living with type 1 diabetes.